1. Field of the Invention
The present invention relates to a signal controlling apparatus and method in a WDM optical communications system.
2. Description of the Related Art
In recent years, the speed and the capacity of an apparatus that makes a long-haul international communication like an optical submarine terminal apparatus have been increasing rather quickly with WDM technology. Accompanying this phenomenon, the demand for some control functions for maintaining a main signal characteristic has been growing. The first one is a supervisory control function of a submarine repeater or gain equalizer, which is installed on a submarine transmission line. The second one is an automatic pre-emphasis control function for making wavelength characteristics uniform. The third one is an automatic threshold value control function for adjusting an identifing voltage/phase, which shifts with fluctuations of a Q value of a transmission line or fluctuations of a temperature of a receiver, to an optimum value. The fourth one is an automatic dispersion compensation function for compensating for wavelength dispersion with a variable device. These control functions include a function for performing control at the start-up of an apparatus, a function for periodically performing control during operations, and the like. With these controls, however, the level of a main signal is varied or modulated. Therefore, if the individual controls are performed at the same time, a signal characteristic can possibly be degraded in a transient manner, leading to an occurrence of an error.
FIG. 1 exemplifies the configuration of a conventional WDM optical submarine terminal apparatus.
This figure shows an opposed connection between a station A (terminal station 10) and a station B (terminal station 11). Firstly, a TRIB (TRIButary) unit 12 of the station A is, what is called, a transponder unit, and a functioning unit that performs a signal process for one wavelength. Accordingly, TRIB units 12 the number of which is equal to the number of wavelengths are arranged in the terminal station 10 in the WDM system. However, FIG. 1 shows only the TRIB unit 12 for one wavelength. To a TXF (transmitter unit) 18, a SDH signal, etc. from a land is input. The TXF 18 once opt-electrically converts the signal, etc., adds a Reed-Solomon code for error correction, electro-optically converts the signal, and outputs the signal. Then, a CHA (CHannel Amplifier) unit 17 at the next stage amplifies the optical signal from the TXF 10 to a predetermined level. Then, the signal enters a DCF (Dispersion Compensation Fiber) 50, where pre wavelength dispersion compensation is performed for this signal. A TPA (Transmitting Pre-emphasis Amplifier) 16 at the next stage has a pre-emphasis function for adjusting the level of an optical signal after being amplified so that the characteristic of each wavelength (each channel) becomes flat after being transmitted. The above described TRIB units 12 are arranged by the number of wavelengths to be multiplexed, and wavelengths output from the TRIB units 12 are multiplexed by a WDM unit 30, and transmitted to a submarine transmission line.
On the submarine transmission line, optical amplification repeaters 31-1 and 31-2 are inserted. Also gain equalizers 31-3 and 31-4 for equalizing the gain of each wavelength are inserted depending on the number of multiplexed wavelengths or a transmission distance.
The above described signal is received and wavelength-demultiplexed by a WDM unit 32 of the station B (terminal station 11). After post dispersion compensation is made for a signal having each demultiplexed wavelength by a DCF 33, it is amplified by a CHA unit 34 similar to the transmitting side, and input to a RXF (receiver unit) 35. The RXF 35 opt-electrically converts the received signal, demodulates the Reed-Solomon code for error correction, makes error correction, electro-optically converts the signal into a SDH signal, and transmits the signal to a land transmission line.
The above description is the flow of a main signal. MPTRs (Management Processors for TRibutary) 23 and 41 within the TRIB units 12 and 14 warn, supervise, and control the respective units. COM units 13 and 15 interface with the MPTRs 23 and 41 within the TRIB units 12 and 14, and supervise and control the whole of the terminal apparatuses. Additionally, the COM units 13 and 15 are connected to CTs (Craft Terminals) 29 having GUI and 47, and SSEs (System Supervisory Equipments) 18 and 46.
Each of the COM units 13 and 15 is arranged for all of wavelengths, and a signal from the MPTR 23 or 41 of the TRIB unit 12 or 14 for each wavelength is passed to a SMP (System Management Processor) 27 or 45 via a SIP (Signal Interface Processor) 24 or 42 of the COM unit 13 or 15. To the SMPs 27 and 45, PCPs (Pre-emphasis Control Processors) 25 and 43, and RSPs (Repeater Supervisory Processors) 26 and 44 are connected in addition to the SSEs 28 and 46, and the CTs 29 and 47.
Conventionally, a supervisory control of a submarine repeater/gain equalizer, and a pre-emphasis automatic control are normally implemented as controls of a main signal.
With a method of the supervisory control of a submarine repeater/gain equalizer, a supervisory control command is first transmitted from the SSE 28 or 46, or the CT 29 or 47 to the SMP 27 or 45. The SMP 27 or 45 transfers the command to the RSP 26 or 44, which then transmits the command to the WDM unit 30 or 32. The WDM unit 30 or 32 superposes the received command signal on a signal after being wavelength-multiplexed, and transmits the signal. Commands include a control command for varying the output level of a repeater, and a monitor command for monitoring an input/output level, a pump LD current (an electric current controlling a pump light source), etc. Any of the commands are superposed in the same manner. Normally, a modulation frequency is a low frequency of approximately 150 kHz, and its modulation factor is approximately 5%, which exerts less influence on a main signal.
With a method of the pre-emphasis automatic control, an optimum pre-emphasis setting value of each wavelength is calculated and set by monitoring the number of error corrections of each wavelength on a receiving side of an opposed station.
If a pre-emphasis control command is issued from the SSE 46 (CT 47) of the station B (terminal station 11), the MPTR 41 of the station B (terminal station 11) receives the number of error corrections of the TRIB unit 12 of the station A (terminal station 10) (the number of error corrections is obtained by the RXF 20). Specifically, the number of error corrections made by the RXF 20 of the station A (terminal station 10) is transmitted to the MPTR 23 of the station A (terminal station 10), and its information is obtained by a FOI (FEC Overhead Interface) 19. The FOI 19 inserts this information in an overhead for error correction. Then, this signal is transmitted from the TXF 18 of the station A as a main signal. In the station B (terminal station 11), the information of the number of error corrections of the station A from the signal received by the RXF 35 is transmitted to the MPTR 41 via the FOI 36.
The COM unit 15 of the station B obtains the number of error corrections of the opposed station from the MPTR 41 of a TRIB unit 14 for each wavelength. The information of the number of error corrections is obtained by the PCP 43. The PCP 43 converts the number of error corrections into a Q value for each wavelength, and calculates for each wavelength a pre-emphasis setting which equalizes Q values of wavelengths as much as possible. Then, the calculated value is set in a TPA 40 of each of the TRIB units 14, and a Q value is recalculated. Here, if the Q values of the wavelengths are not equal, the PCP 43 recalculates and resets the Q values. This routine is repeated by several times, so that a difference among the Q values of the wavelengths is gradually converged, and the Q values are finally settled into an optimum value. Here, the pre-emphasis automatic control is completed.
The SSE 28 and the CT 29 of the terminal station 10 are connected to the SSE 46 and the CT 47 of the terminal station 11 via an electric transmission line different from the optical transmission line so that they can exchange information required for the supervisory control of a main signal. Typically, this electric transmission line is an electric line having a transmission rate of 2 Mbps.
Recently, the demand for adopting the following control functions in addition to the above described two controls has been arising due to an increase in the number of wavelengths, speed-up of a transmission rate, etc.    an automatic adjustment function for amplitude and phase threshold values of a receiver    an automatic adjustment function for a dispersion compensation amount
The former function is demanded, since there is a concern such that an ability originally delivered as an error correction function cannot be fully implemented due to Q value fluctuations of a transmission line or fluctuations of a device temperature, which is caused by an increase in the number of wavelengths or an improvement in a transmission rate, etc. This is because a threshold value in an amplitude direction and that in a phase direction are shifted, when a received waveform is reproduced by a receiver. Accordingly, these threshold values must be automatically adjusted to optimum points with an external trigger.
Additionally, the latter function is demanded, since there are difficulties in a fine adjustment of a dispersion compensation amount and the downsizing of a device with the use of a DCF (Disperson Compensation Fiber). This is because a rather large installment area is required for dispersion compensation that is conventionally made with the DCF whose dispersion compensation amount is fixed. Accordingly, there is a demand for possessing a function that can vary a dispersion compensation amount according to a voltage value with the use of a VIPA (Virtually Imaged Phased Array) device that has been developed recently. Therefore, a function for automatically adjusting the dispersion compensation value with an external trigger becomes necessary.
Fundamentally, these control functions superpose a supervisory signal on a main signal, and vary a main signal level, a threshold value or a dispersion compensation value, etc. Therefore, if these controls are executed at the same time, a signal characteristic can possibly exceed a tolerable range, leading to a degradation and danger.